This invention generally relates to processes for capturing carbon dioxide (CO2) from gas streams which contain a mixture of constituents.
The emission of carbon dioxide into the atmosphere from industrial sources such as power plants is now considered to be a principal cause of the “greenhouse effect”, which contributes to global warming. In response, tremendous efforts are underway to reduce emissions of CO2. Many different processes have been developed to attempt to accomplish this task. Examples include polymer and inorganic membrane permeation; removal of CO2 by adsorbents such as molecular sieves; cryogenic separation; and scrubbing with a solvent that is chemically reactive with CO2, or which has a physical affinity for the gas.
One technique has received much attention for removing CO2 from flue gas streams, e.g., exhaust gas produced at power plants. In this technique, aqueous monoethanolamine (MEA) or hindered amines like methyldiethanolamine (MDEA) and 2-amino-2-methyl-1-propanol (AMP) are employed as the solvents in an absorption/stripping type of regenerative process. The technique has been used commercially for CO2 capture from coal fired power plants and gas turbines.
There are certainly considerable advantages inherent in the MEA and hindered amine-based absorption processes. However, a number of deficiencies may be preventing wider adoption of this type of technology. For example, the process can sometimes result in sharp increases in the viscosity of the liquid absorbent, which can cause clogging of pipelines. To avoid this problem, the concentration of MEA and other amines is sometimes maintained at a relatively low level, e.g., below about 30 wt. % in the case of MEA. However, the lower concentrations can greatly reduce absorbing capacity, as compared to the theoretical capacity of the neat absorbent.
Moreover, energy consumption in the MEA process can be quite high, due in large part to the need for solvent (e.g., water) heating and evaporation. For example, the process may consume about 10-30% of the steam generated in a boiler that is heated by combustion of a fossil fuel. Furthermore, MEA-based absorption systems may not have the long-term thermal stability, in the presence of oxygen, in environments where regeneration temperatures typically reach at least about 120° C.
Additional drawbacks may result from the fact that the liquid absorbent which is enriched with CO2 in the MEA or hindered amine process may still contain a substantial amount of free amine and solvent (usually water). The amine and water are moved in the vapor phase under thermal desorption, but can cause corrosion and other degradation in the attendant equipment. To address this concern, specialized, corrosion-equipment materials can be used for the equipment, but this can in turn increase capital costs for the plant. In some cases, corrosion inhibitors can be added, but the use of these specialized additives can also increase operational costs. Moreover, the oxidation of the MEA or hindered amine absorbents can acidify some of the solvents present. In addition to the corrosion problems which can result, this may decrease the available alkalinity for CO2 capture, thereby reducing process efficiency.
Another example of a commercial CO2 post-combustion capture process uses aqueous solutions of piperazine-promoted potassium carbonate (K2CO3). However, this process is often very energy-intensive, and can be economically inferior to the MEA process. Still another example involves the use of chilled ammonia. In this case, energy-intensive cooling systems are usually required for such a system, and the risks associated with unintended ammonia release may be unacceptable.
In view of these considerations, new methods for treating gas streams which contain CO2 would be welcome in the art. The new processes should effectively remove some portion of CO2 from the gas stream, under conditions which are economically viable in some industrial settings. Moreover, the processes should be compatible with related systems, e.g., power generation systems based on gasification, combustion, and the like.